The Image Quality Of Megavoltage Cone-beam Computed Tomography And Its Application On Dose Calculation

The applications of 3D conformal radiotherapy (3DCRT) and intensity modulated radiation therapy (IMRT) improved the dose gradients between the target volumes and the normal tissues. These more conformal, higher dose treatments can decrease both the spread of disease and normal tissue complication. The precision of patient positions and organ motions become more important. So, it needs the acquiring of actual images on the treatment table. Megavoltage cone beam computed tomography (MVCBCT) can obtain the patient's 3D information just on the treatment table, and may improve the position accuracy after registering with the planning CT images. The main application of MVCBCT is used in image-guided radiation therapy. But such technique can't provide the actual dose that patient received during the treatment. Estimate the dose delivered in previous fractions and optimize the treatment for future fractions, as so called dose-guided radiation therapy. It is a hot research spot both at home and abroad. The purpose of the study is to obtain the optimum imaging protocols and the feasibility of using MVCBCT images for patient's dose verification. This is the first domestic study of MVCBCT system in clinical applications.The stability and safety is the premise of using MVCBCT for clinical use. The thesis began with the study on establishing the quality assurance (QA) procedures of MVCBCT system. The protocols of daily QA, monthly QA and annual QA was created according to the system commission and the experiences of our department. With this protocol , the system maintain a good perform.Based on the complete QA, the thesis studied some clinical applications of using MVCBCT images. The physics of radiation interactions in the megavoltage energy range may force compromises between the dose delivered and the image quality, in comparison with conventional CT. The thesis studied the image quality with the different imaging doses and gave some guidelines for clinical applications in head and neck patients,and studied the possibility of using MVCBCT images for dose recalculation were evaluated on the head and neck patients. The usage of MVCBCT images for monitoring the dose changes was also investigated in patients with anatomical or positional changes. The study showed that 1) the air cavity, bone and soft tissue can be distinguished using 5-7 monitor units (MU); and 13-15MUs to distinguish some difference soft tissues. 2) The MVCBCT images can be used for dose recalculation with a good accuracy, which was less than 3% and 3mm compared with planning CT in patients for validation. 3) The feasibility of using patient's MVCBCT images just acquired before beam delivery for dose verification.